Abstract
AbstractTop‐down estimates of CO2 fluxes are typically constrained by either surface‐based or space‐based CO2 observations. Both of these measurement types have spatial and temporal gaps in observational coverage that can lead to differences in inferred fluxes. Assimilating both surface‐based and space‐based measurements concurrently in a flux inversion framework improves observational coverage and reduces sampling related artifacts. This study examines the consistency of flux constraints provided by these different observations and the potential to combine them by performing a series of 6‐year (2010–2015) CO2 flux inversions. Flux inversions are performed assimilating surface‐based measurements from the in situ and flask network, measurements from the Total Carbon Column Observing Network (TCCON), and space‐based measurements from the Greenhouse Gases Observing Satellite (GOSAT), or all three data sets combined. Combining the data sets results in more precise flux estimates for subcontinental regions relative to any of the data sets alone. Combining the data sets also improves the accuracy of the posterior fluxes, based on reduced root‐mean‐square differences between posterior flux‐simulated CO2 and aircraft‐based CO2 over midlatitude regions (0.33–0.56 ppm) in comparison to GOSAT (0.37–0.61 ppm), TCCON (0.50–0.68 ppm), or in situ and flask measurements (0.46–0.56 ppm) alone. These results suggest that surface‐based and GOSAT measurements give complementary constraints on CO2 fluxes in the northern extratropics and can be combined in flux inversions to improve constraints on regional fluxes. This stands in contrast with many earlier attempts to combine these data sets and suggests that improvements in the NASA Atmospheric CO2 Observations from Space (ACOS) retrieval algorithm have significantly improved the consistency of space‐based and surface‐based flux constraints.
Highlights
Observations of atmospheric CO2 provide a constraint on the net surface‐atmosphere CO2 flux and are critical for monitoring carbon flux changes
Large spatial structures in the posterior‐simulated CO2 fields are compared with Gases Observing Satellite (GOSAT) and Orbiting Carbon Observatory 2 (OCO‐2) XCO2, while the accuracy of the fluxes is evaluated against aircraft‐based CO2 measurements
The CO2 fields simulated with the prior fluxes tend to be biased low relative to GOSAT and OCO‐2 during the winter and spring and biased high during the summer and fall in the northern extratropics, suggesting that the prior fluxes underestimate the magnitude of the seasonal cycle
Summary
Observations of atmospheric CO2 provide a constraint on the net surface‐atmosphere CO2 flux and are critical for monitoring carbon flux changes This has motivated observational programs that measure atmospheric CO2, including a global network of surface‐based in situ and flask monitoring sites, the Total Carbon Column Observing Network (TCCON) of ground‐based spectrometers (Wunch et al, 2011) and several satellite missions (Crisp et al, 2004; Yokota et al, 2009). Journal of Geophysical Research: Atmospheres current measurement programs are unable to continuously monitor CO2 with global coverage, resulting in observational gaps These spatial and temporal gaps in observations of atmospheric CO2 can introduce artifacts into NEE estimates, leading to difficulties in constraining carbon fluxes on regional scales (Basu et al, 2018; Byrne et al, 2017; Collatz et al, 2014)
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